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Mar/Apr 2012 | |
Ultrafine carbides key to more durable microendmills
Submicron-grade carbides create dense endmill bodies that resist breakage, but so far making metalcutting endmills with carbide grains smaller than 0.4μm has been a challenge.
Ultrafine grades are also the key to developing ever smaller micro-endmills with smaller, sharper and more consistent features (the carbide grain size has to be at least slightly smaller than a tool’s features). Submicron carbide grades are typically made from grains 0.5μm to 0.8μm and ultrafine grades range from 0.2μm to 0.5μm.
While submicron and ultrafine carbide grades have been commercially available for years, the practical lower limit on grain size for metalcutting tools appears to be 0.4μm. Some toolmakers cite problems with grain consistency (larger particles mixed with smaller ones) and grain growth during the sintering process with particles under 0.4μm.
Exceptional combination
Ultrafine tungsten carbides have provided microtool manufacturers with the hardness and toughness critical to successful microtool applications, according to Aharon Inspektor, senior staff engineer, global grade development, Kennametal Inc., Latrobe, Pa.
Because the physical dimensions of the micro-edge are often comparable to the size of the machined features and to the grain size of the workpiece materials, many standard carbide grades are inadequate for microtools, Inspektor said. By using ultrafine-grain tungsten carbides, a tool manufacturer can modify the microstructure of the cutting edge and fine tune the balance between the edge geometry and the tool material.
Breaking it down: Tips for avoiding tool breakage when micro-endmillingBreakage is a common problem for micro-endmill users, particularly with difficult-to-machine materials such as titanium. MICROmanufacturing asked several microtool experts for their advice on how machinists can avoid tool breakage when micro-endmilling. The following are edited excerpts of their comments. Joe Negron, international sales manager, Kyocera Micro Tools, Costa Mesa, Calif.: One of the biggest causes of breakage is misapplication of the tool. Say you have a flute length of 0.070" and you have 0.060" of the flute length engaged in the workpiece, you’re going to have higher breakage. You have to find a proper flute length and stay safely within 50 to 60 percent of the flute length. Also, microtools need to be optimized for specific industries in order to reduce breakage. For example, take high-volume intraocular lens manufacturers that are cutting soft plastics. We’ve worked with several manufacturers to optimize tooling. We create several tool prototypes and test them to determine factors such as what is the optimum core diameter for tools cutting soft plastics. Dave Burton, president and general manager, Performance Micro Tool, Janesville, Wis.: One of the biggest factors in tool breakage is using inadequate spindle speeds. You need to use the highest available spindle speeds while minimizing vibration and runout. Our smallest tools are 5µm in diameter and they have been run as low as 50,000 rpm in stainless steel, but shops typically have greater success with 100,000-plus rpm. That typically requires a newer machine designed specifically for micromachining, including the new tabletop micro machine tools. More milling machines with high-speed spindles and low vibration and runout are becoming available, which allows micro-endmills to be used more effectively. Jeff Davis, vice president of engineering, Harvey Tool Co. LLC, Rowley, Mass.: Micro-endmills are very fragile. You have all the same concerns as you do in macromachining, just to a much greater extent. We look at runout, speeds and feeds, toolholding, capability of the machining center, programming and coolant. In macromachining, runout is a concern, but the problem of tool breakage due to runout is less critical. You can use a ½"-dia. mill that has some runout and you aren’t going to be very concerned, but with microtools you have to be concerned about the total runout of the entire setup—cutter, toolholder and spindle. If you have a 1/16"-dia. mill with 0.001" runout, you will most likely end up popping that tool. Users have to evaluate their machines. Many times, they are running a machine that’s not designed for miniature milling. For example, they’ll try to use a machine with just 5,000 or 6,000 rpm that is not very rigid or stiff. Ideally, you should use a machining center that has very high stiffness, rigidity, no chatter and has a higher rpm. It must also be capable of rapid acceleration and deceleration so it can avoid interrupted cuts. Speeds and feeds are chosen based on the cutter diameter and the material you’re cutting, but in general in micromachining it’s better to have a machine that has at least 10,000 rpm. Software is also critical. If you have a toolpath that goes directly into a corner and produces extra engagement, you’re likely to break the tool in the corner because of that additional pressure. The software needs to look-ahead and see that corner coming up. In standard programming, if you’ve got a corner that has a ¼" radius, you might use a ½"-dia. mill and bring it into the corner and then pull out of the corner and you’d be left with your ¼" radius. In micromachining, you don’t want to do that. Say you’re allowed a 1/32" radius, so you could potentially use a 1/16" mill to pull in and pull out, but I wouldn’t recommend that. Instead of a 1/16" (0.0625") mill, why not use a 0.050"-dia. or a 0.040"-dia. mill, where you are swinging through the radius. You’re not coming to a stop and then starting again, which lessens the risk of tool breakage. If you have a long length-of-cut scenario or even a long-reach scenario, you don’t want the tool to be any longer than it has to be in order to avoid breakage. For example, Harvey Tool offers a line of microtools that has three different reaches: 5-times, 8-times and 12-times diameter. Why use a 12-times reach for a 5-times deep pocket? Ideally, you want the tool to be just a whisker longer than is necessary for your application. Jonathan Hay, vice president, sales and marketing, US Union Tool, Buena Park, Calif., offered a list of dos and don’ts for avoiding tool breakage. Do:
Don’t Use:
Joseph Hall, technical sales representative, inspection applications, Zoller Inc., Ann Arbor, Mich.: With respect to tool breakage, tool presetters help in two ways: providing the precise X (width) and Z (length) of the outermost position of a tool in space. Inspection machines provide precise dimensional results of features, such as the rake angle on a helical tool (which can cause breakage if it’s too aggressive, for example). Any tool introduced to a cutting surface too aggressively will fail. Or, more exactly, if the CNC milling center moves the tool to a point in space occupied by the workpiece, the tool will break. For this reason, most programmers will move the tool “close” to the workpiece and then slowly move the now-rotating tool into the workpiece. When the exact (within a couple microns) dimensions of the tool are known, the tool can be positioned more closely and therefore more quickly and efficiently. All Zoller machines (not just inspection machines) help with this by providing the precise X and Z coordinates of the tool using either a spot measurement or the CRIS function, which rotates the tool 360° and provides a silhouette of what will actually be cut—an important feature due to the variety of holding methods. Regardless of how the tool is held, this function determines exactly what will happen when the tool is used—and can therefore reduce the chance of breakage due to positioning problems. Physical characteristics of the tool are also key breakage factors If the rake angle, helix angle or other feature of the tool is too aggressive, the tool will fail. Aharon Inspektor, senior staff engineer, Global Grade Development, Kennametal Inc., Latrobe, Pa.: Micromachining presents several specific challenges for the tool, such as the high ratio between cutting force and diameter of the tool (which would accelerate breakage), low surface speed (buildup, increased forces and breakage) and accelerated effect of tool wear (plowing instead of shearing and tool deformation). Even as the nominal rotational spindle speed is high, the actual surface cutting speed is lower than in conventional machining. The low cutting speed would accelerate buildup, leading to rounded edges and burr formation. In general, a high feed increases the thrust force and feed rates should be reduced for small tools, but watch for changes in chip thickness. Change in chip size and flow could be an early indication for rounding of the cutting edge. Dick DeVor, research professor, department of mechanical science and engineering, University of Illinois, Urbana-Champaign:The worst thing you can do is choose a feedrate that is too low because you have the problem of the feedrate being of the same order of magnitude of the cutting tool’s edge radius. This brings the minimum chip thickness effect into play where you’re plowing more than shearing, which produces higher cutting forces and can lead to chatter, instability and a poor surface finish. We have done a great deal of research to develop ways to predict this minimum chip thickness effect based on the material properties. We have found that cutting at feedrates one and one-half to two times the minimum chip thickness may be necessary to avoid this problem. —A.Rooks |
However, using an ultrafine-grain carbide by itself does not solve the problem of tool breakage, Inspektor said. “Raising hardness of the tool substrate will increase rigidity that, by itself, may even accelerate breakage,” he said. “To reduce breakage, you need to increase the toughness of the tool by adding binder; minimize cutting force by, for example, creating a sharp cutting edge; and provide easy chip flow through good geometry and smooth surface finish.”
The 0.4μm solution
There are several problems with carbide powders having grain sizes under 0.4μm, according to Jim Oakes, vice president, technology- tungsten metals, ATI Engineered Products, Huntsville, Ala. “There are issues with powder manufacturing technologies, the use of grain growth inhibitors and sintering technology,” he said. “During the liquid phase of sintering, some of the tungsten dissolves and goes into the cobalt, and as it cools the tungsten comes back out and grows on other carbide grains.” Grain growth changes the mechanical properties of the tool and can lead to unpredictable tool performance. Some toolmakers are using microwave technology to speed the sintering process and limit grain growth, he said. Additives such as vanadium carbide and chromium carbide can also slow grain growth.
A consistent sub-0.4μm ultrafine carbide powder that does not promote grain growth would be a major step toward reducing endmill breakage, according to Frank Pfeff erkorn, assistant professor, mechanical engineering, University of Wisconsin-Madison. “With micro-endmills, [where] some of the features are close to the grain size, there is some randomness in tool life based on whether the grain boundary is right at the sharp edge or if it is between two grains,” he said. “If it is the latter, that cutting edge will be a little weaker and more susceptible to breakage. Using a finer grain size for the given features would off er more consistent results or, if a crack does form, there would be more pathways for the crack to deviate sideways, or slow down.”
Tool wear and cutting edge chipping lead to higher cutting forces, which at some point lead to tool failure. In his work on micro-endmills, Pfeff erkorn has observed that failure often occurs at the point on the tool where the fl utes begin. “There is some kind of stress concentration there,” he said.
Waiting for a 0.2μm carbide Dave Burton, president and general manager of Performance Micro Tool, Janesville, Wis., agreed that smaller grains are critical to creating more durable micro-endmills. According to Burton, his company produces 0.0002"- dia. endmills and needs a smaller grain size to make a 0.0001"-dia. endmill.
“We have been hoping that a 0.2μm grain size would be right around the corner,” he said. “For the past 5 years, manufacturers have been saying they are close or they’ve got it, but when you try it they don’t really have it. The batches contain some 0.2μm grain sizes, but bigger grains [are] in there as well, and that makes it very unpredictable.”
The core of the smallest tool that Performance Micro Tool makes is 2μm in diameter. “That means there are five grains of carbide holding that tool together,” he said. “If they can cut that grain size in half and 10 grains are available to hold the tool together, not only can I make a better tool, I might be able to cut my tool size from a diameter of 5μm to 3μm—even 2μm.”
Joe Negron, international sales manager, Kyocera Micro Tools, Costa Mesa, Calif., agreed that ultrafine carbide grades are critical for microendmills—not only to reduce the potential for breakage but to improve tool performance. “With a denser tool, the cutting edge will wear more evenly,” he said. “A coarse carbide will chip pretty badly as it is wearing.” He added that a smoother tool finish can reduce breakage and improve tool life [by helping with chip evacuation and reducing avenues for crack propogation]. To produce mirror finishes on its microendmills, Kyocera uses a superfine, 1,200-grit or higher grinding wheel.
Negron also stressed the need to reduce the potential for breakage by optimizing the endmill for a specific market application—particularly by varying the endmill’s core diameter, defined as the percentage of the tool diameter that is not part of the flute. For example, if a tool will be used on harder materials, the core diameter may need to be as much as 60 percent of the diameter, while for softer materials, a core diameter of 50 percent may be sufficient.
Alistair Grearson, senior R&D specialist, Sandvik Hard Materials, Coventry, England, agreed that the “holy grail” of a 0.2μm ultrafine carbide grade for metalcutting applications remains elusive. “While the minimum grain size limit in conventionally produced hard metals is around 0.2μm, this grain size gives too much brittleness for metalcutting,” said Grearson. Metalcutting grades generally start at 0.3μm, but are mostly in the range of 0.5μm to 0.9μm. He said the grain size of SHM’s metalcutting rod grades for microtools range from 0.3μm (grade PN90) to 0.5μm (grade DM80) to 0.7μm (grades H10F and H12F).
Attacking the problem
Carbide producers are attacking the problems of grain growth during sintering, inconsistent grain sizes in the powder and difficulty in measuring grain sizes to assure consistency, according to Uwe Schleinkofer, manager of R&D, cutting tools, Ceratizit Austria (Tirol) GmbH.
“There are two approaches to dealing with these problems,” he said. “One is strict control of the starting materials [to ensure there is] a very narrow grain size distribution (measurement below 0.5μm still is a challenge at the moment), and the second is optimization of doping to avoid grain growth during sintering and adjustment of sintering conditions.” According to Schleinkofer, the use of FSS (Fisher Subsieve Sizer) measurement is not eff ective in this grain size range, and he recommends using the BET specific surface area (Brunauer, Emmett and Teller) method to determine the powder quality. However, this method still does not supply any information about the grain size distribution of the powder.
So when will 0.2μm grain carbides for metalcutting tools be available? The key is controlling grain growth, which will require new inhibitors and improved sintering furnaces, said Oakes of ATI. “I would expect to see the improvements needed for robust sintering processes for these materials to occur in the next 5 to 10 years,” he said.
About the author: Alan Rooks is editorial director of MICROmanufacturing. Telephone: (847) 714-0174. E-mail: arooks@jwr.com.
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